In U.S. Pat. No. 4,468,623 to Gianzero et al a tool and technique are described to make fine detailed resistivity investigations from which a fine display can be formed in a manner as shown and described in U.S. Pat. No. 4,567,759 to Ekstrom and Chan. The tool has a plurality of pads pressed against the borehole wall. Each pad carries an array of small electrodes. The array includes multiple, vertically spaced rows of electrodes with the electrodes of different rows arranged so as to provide, in the aggregate, an overlap as viewed along a circumferential direction. This overlap is desired to reduce visually disturbing, sawtooth type, aliasing effects (laterally distributed artifacts) when measurements obtained from the electrodes are recorded as a display.
However, when electrodes in different vertically displaced rows are used, precise depth shifting of measurements is needed to combine the measurements of one row obtained at one depth to those of another row obtained at the same depth. Although the rows are close together so that the amount of depth shifting is small, problems relating measurements at one depth to another still arise because of unusual tool movements. Special techniques are employed to provide so called "speed corrections" and can be quite effective, see for example U.S. Pat. No. 4,545,242 to Chan.
Canadian Patent No. 685,727 to Mann et al proposes a lateral array of small electrodes on a pad. Each electrode introduces, in sequence, a separately measurable survey current for an electrical investigation of the earth formation. The electrodes are placed in a single horizontal row with circumferential spacings between electrodes and a device is used to sequentially excite and measure survey currents from the electrodes.